1. Field of the Invention
The present invention concerns an x-ray converter element.
2. Description of the Prior Art
An x-ray converter element is a component of a digital detector for radiography that is, for example, described in the article by R. F. Schulz, “Digitale Detektorsysteme für die Projektionsradiographie” in Fortschr. Röntgenstr. (2001) 173, pages 1137 to 1146, in particular illustration 5.
The x-ray converter element (that includes a scintillator) is conventionally used in combination with a CCD camera (CCD—charge-coupled device). Most of the incident x-ray radiation is absorbed in the scintillator (luminescent screen) and converted into visible light. The light image is projected onto the CCD camera with the aid of imaging optics (for example optical lenses, mirrors, prisms etc.). The CCD camera transduces the light image into electrical signals. The electrical signals are further processed and are output as a digital image.
This detector system has the advantage that it is composed only of relatively easily obtainable and inexpensive components and therefore can be produced in a relatively cost-effective manner. Such a detector system thus represents a relatively cost-effective solution with the known advantages of a digital detector system (film-less, image processing etc.).
The dose requirement in such a detector system is comparable to the dose requirement of classical film-foil systems. The dose savings that is possible to achieve with a planar image detector (flat panel detector) cannot be achieved with this detector system. Flat panel detectors are described in the article by M. Spahn et al. “Flachbilddetektoren in der Röntgendiagnostik” in “Der Radiologe 43 (2003)”, pages 340 through 350.
The reason for the relatively high dose requirement in radiography is the occurrence of phenomena known as “secondary quantum sinks”.
The signal-to-noise ratio of an ideal x-ray detector (negligible electronic noise, no structural noise) is defined by the number of the absorbed x-ray quanta and is designated as a “primary quantum sink”.
In a flat panel detector an absorbed x-ray quantum is transduced into, for example, 1000 electrons. The additional statistical electron noise thus is negligible due to the relatively large number of electrons.
However, when fewer than 10 electrons are generated per x-ray quantum, this additional noise is no longer negligible and impairs the image quality or increases the dose requirement. This is designated as a “secondary quantum sink”. This is explained in the publication by R. M. Gagne et al., “Optically coupled digital radiography: sources of inefficiency” in “Processing. SPIE Vol. 4320 (2001)”, pages 156 to 162.
It is a basic requirement of the scintillator that every x-ray quantum striking the scintillator should generate an optimally large number of light quanta in the scintillator that must in turn be optimally transduced into electrons without loss.
A further requirement of the scintillator is mechanical stability. For installation the scintillator is fixed only at its outer edge. The relatively large x-ray converter element (for example 44 cm×44 cm) can oscillate similar to a drum head. During operation, and even more during transport, the x-ray detector is exposed to significant shocks and vibrations, for example given freight vehicle or rail transport.
In order to ensure that the scintillator is not damaged by such oscillations, URL http://sales.hamamatsu.com/assets/pdf/parts_J/ALS_ACS_FOS.pdf) discloses using use converter layers in which case the scintillator made from Cdl:Tl is applied either on a 1 mm-thick carrier made from aluminum, or on a 2 mm-thick carrier made from amorphous carbon. In order to achieve a sufficient mechanical stability, care must be taken that these layer thickness are observed as a minimum.
Since the carrier is arranged in the beam path in front of the scintillator, however, it acts as a ray filter. The energy that is absorbed in the carrier is therefore no longer available for light generation in the scintillator.
The transmission for x-ray radiation given 2 mm amorphous carbon is good, only slightly reducing the transmission for x-ray energies smaller than 30 keV. A disadvantage of the use of amorphous carbon is its high price.
In contrast to this, aluminum is a low-priced material. A disadvantage in the use of aluminum is its relatively low transmission for x-ray energies less than 40 keV.